1. Field of the Invention
The present invention relates to printed circuit boards (PCBs), and more particularly, to land patterns and associated routing in PCBs.
2. Background Art
Integrated circuit (IC) chips or dies from semiconductor wafers are typically interfaced with other circuits using a package that can be attached to a printed circuit board (PCB). One such type of IC die package is a ball grid array (BGA) package. BGA packages provide for smaller footprints than many other package solutions available today. A BGA package has an array of solder ball pads located on a bottom external surface of a package substrate. Solder balls are attached to the solder ball pads. The solder balls are reflowed to attach the package to the PCB. BGA packages are available in a variety of types, including plastic BGA (PBGA) packages, flex BGA packages, fine pitch BGA (FPBGA or FBGA) packages, and wafer-level BGA (WLBGA) packages, for example.
BGA packages are typically mounted to a PCB by attaching the package to a land pattern formed in a top routing layer of the PCB. The land pattern includes a plurality of electrically conductive land pads arranged in an array of rows and columns. Each solder ball on the bottom of the BGA package attaches to a corresponding land pad of the land pattern when the BGA package is mounted to the PCB. PCBs typically include multiple electrically conductive routing layers that have traces formed therein to route signals from the land pads to other locations of the PCB. For example, traces are typically formed in the top routing layer of the PCB to route out the first couple of peripheral rows of the array. Traces that are routed from land pads of the peripheral edges of the array do not need to pass between other land pads. Traces that are routed from land pads that are internal to the array (e.g., not in the peripheral edges) are typically routed between land pads located in the periphery of the array to reach external to the array.
BGA package sizes are shrinking as the demand for smaller and smaller electronic devices increases. As such, the sizes of the BGA package land patterns are also shrinking, making it increasingly more difficult to form routing in PCBs for the land patterns. For example, in some cases, land pads must be moved so close together that traces cannot be routed between them. In such land patterns, only land pads in the peripheral edges of the land pattern can be routed external to the array on the top layer of the PCB. Accordingly, additional routing layers must be added to the PCB to be able to fully route the land pattern. The additional routing layers add to the cost and complexity of PCBs.
Thus, what are needed are improved circuit boards and routing techniques that can accommodate increasingly smaller BGA packages.